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Stephanie Burnett Heyes and Chii Fen Hiu, Research Fellow at the British Academy and lecturer at the University of Birmingham; Department of Experimental Psychology, University of Oxford
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Adolescence, the period of life between childhood and adulthood, is defined by the transition from parental dependence to relative autonomy. During this time, important changes take place in the structure and workings of the brain, and in the mental abilities which underlie some of the most sophisticated human behaviours.

One of the central events of adolescence is puberty, the transition to reproductive maturity. We now know that puberty impacts not just the production of gametes, but is also associated with changes in the brain and in behaviour. Of course, adolescence is also a socio-cultural phenomenon. Across time and place, its duration varies widely, as do its “typical” behaviours. Yet studies conducted with humans and with non-human animals suggest there may be some biological commonalities of adolescence reflecting changes in brain and behaviour. By all accounts, adolescence happens for a reason.

In humans, adolescence is often characterised as a risk factor in itself, something to be managed or controlled. However, current scientific thinking highlights a series of carefully orchestrated, exquisitely sensitive neurodevelopmental changes that make adolescence a period of both vulnerability and opportunity.

This piece will discuss some of the very real risks of adolescence, such as vulnerability to mental illness, alongside the unique plasticity that allows the adolescent to flexibly adapt to changing socio-cultural demands. Finally, we will consider what can be done to support optimal adolescent development and experience.

Adolescent brain development

The human brain continues to mature throughout childhood and adolescence. In many regions of the brain, the number of connections increases sharply in the first few months and years of life, and then gradually decreases to adult levels during puberty and adolescence.1 Myelin, the fatty substance that insulates the brain’s axonal conduction fibres and speeds their electrical impulses, increases with age throughout childhood and adolescence.2These changes in synapse density and myelin are thought to correspond to changes across age in the amount of grey and white matter observed using magnetic resonance imaging.

Across different regions of the brain, changes in grey and white matter take place at different rates.3,4Parts of the brain involved in basic sensory processing tend to mature early in life - prior to adolescence. Brain regions involved in higher-level mental abilities, such as language, decision-making, and high-level social understanding, continue to mature during adolescence.

Early maturation of certain brain regions responsive to reward (e.g. subcortical brain regions) and later maturation of brain regions involved in thoughtful, effortful control of behaviour (e.g. prefrontal cortex) is thought to underlie enhanced adolescent reward seeking, exploration and risk-taking.5,6 These same mechanisms may also result in adolescence being a period of enhanced flexibility in learning and adapting to new environments.7

Mental health risk and the impact of the environment

During adolescence, individuals are at their physical peak. However, relative to during childhood, individuals in adolescence are at increased risk of death and disability. This is largely due to theoretically preventable causes such as reckless behaviour and car accidents, in addition to mental illness.8 Indeed, large-scale epidemiology studies have reported that the median age of onset for a range of mental health problems is during adolescence.9,10 According to the World Health Organisation, suicide is the second leading cause of deaths in 15-29 year olds.11

Evidently, the adolescent years play a crucial role in shaping outcomes that have wide-ranging and long-lasting impacts. Importantly, a number of external and intrinsic factors act to shape the course of adolescent development, whether that development is typical or atypical. As discussed below, the developing brain is exquisitely sensitive to its environment.


Abuse and neglect

Abuse and neglect during development are associated with alterations in brain structure.12 This may increase the risk of psychiatric illnesses such as depression and borderline personality disorder in adulthood. Due to the distinct time-course of development across different regions of the brain, the impact of adversity may differ depending on its age of occurrence. In one study, childhood sexual abuse was associated with alterations in subcortical and white matter structures, whereas sexual abuse during adolescence was associated with abnormalities in late-maturing prefrontal cortex.13

Gender

Brain development shows subtle differences across gender. At puberty, some of these gender differences accentuate, and are attributed to the effects of puberty hormones on neural tissue.14 Understanding puberty-linked gender differences in brain maturation may help us to understand the gender imbalance in depression and anxiety, which first arises during adolescence.15

Cannabis
The short and long term impacts of recreational and addictive substances may differ in adolescence compared to adulthood.16 Exposure to high strength cannabis (e.g. skunk) during adolescence may be more detrimental than exposure during adulthood. In a recent study,17patients in South London presenting with first onset psychosis were more likely to have started cannabis during adolescence (<15 years) than were healthy controls.

Another finding from the above study was that, in adults, daily or weekly exposure to high strength cannabis was associated with increased risk of psychotic illness, relative to no cannabis, and relative to weaker forms of cannabis (e.g. hash). It could be beneficial for young people to be made aware of the increased risk of psychotic illness associated with the use of high strength cannabis particularly.

Puberty and nutrition

There is evidence that the age of puberty onset is dependent on both foetal and childhood nutritional intake.18Population-wide changes in nutritional status may therefore impact brain development via mechanisms related to pubertal timing. We do not yet understand the implications of early or late pubertal timing on adolescent brain development, behaviour and mental health risk. Potentially, poor nutrition in utero and during childhood could have mental health consequences many years later.

Culture and ethnicity

Intriguingly, there is evidence that aspects of brain structure may vary across culture. Such differences could potentially reflect the impact of differing linguistic and social environments on the developing brain, although the direction of causality is difficult to determine. For example, one study documented differences between Asian and Caucasian adults in late-maturing regions of the brain, such as the frontal cortex.19

Optimising adolescent development and experience

How can we make use of recent research findings to mitigate the risks of adolescence and promote well-being?

Dietary supplementation

Diet can have a profound impact on the brain and mental health, and represents one means for policy makers to effect positive change. Maternal dietary supplementation with omega-3 fatty acids during pregnancy can improve cognitive indicators of infant brain development shortly after birth.20 Studies are being conducted to evaluate whether omega-3 supplementation during adolescence may alleviate symptoms of emotional disorders.21 More generally, studies have documented a positive relationship between healthy dietary habits and mental health in adolescence, as well as the reverse, above and beyond potential confounding factors such as socioeconomic status and family conflict.22,23

Cultural inclusion

The presentation and recovery rates of many mental health problems vary widely across cultures.24 There are a number of mechanisms that might underlie this disparity, including culturally insensitive diagnostic criteria, variations in social expectations for “normal” behaviour, difficulties in access to mental health facilities, and societal stigma.25 It is important to consider variations in cultural norms when providing support to child and adolescent refugees, who have often experienced trauma and may suffer from mental health disorders.26

More research needs to be conducted outside the typical Western, white, middle class community, to obtain a more accurate picture of global mental health and to develop more inclusive provision of support and treatment worldwide.27

Positive impact of adolescent peer relationships

The impact of peer influence on adolescent risk perception and risky behaviour has been highlighted.28–30 However, peers also have an important and positive role to play for adolescents. Evidence from studies on immigration, refugee status and mental health in adolescence have converged in identifying strong social, including peer, support as a protective factor against adverse outcomes.31–33

Conclusion

Research has shed light on changes occurring during adolescence in the brain, with potential implications for understanding adolescent behaviour and mental health risk. It is our hope that this research will feed into debate on steps that can be taken to optimise adolescent development and well-being worldwide. Across borders and cultures, the provision of accessible mental health services is crucial for society’s young people. As the cornerstone of our future, the world needs to provide adolescents with a solid framework of support, within which they can explore, grow and flourish.

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